- Buyers Guide
Picking PCB Materials for Power Amplifiers
September 24, 2010
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University.
Designing a high frequency power amplifier takes more than just finding the right power transistor. It involves the design of input and output impedance-matching circuits for maximum power transfer and well regulated power-supply circuits. It also requires choosing a printed-circuit-board (PCB) material that will deliver the right combination of performance and reliability for the cost. The right choice of PCB material for an amplifier design can improve gain, gain flatness, gain stability, and output power.
For high frequency amplifiers, the options for PCB substrates are many, from lower-cost FR-4 materials to higher-performance (and higher-cost) PCB substrates materials based on polytetrafluoroethylene (PTFE) dielectric materials. In between these two extremes lie many excellent PCB materials, including some newer thermoset-resin dielectric materials with attractive performance cost ratios. From the low end to the high end, each material selection represents a set of tradeoffs between the quality of physical and electrical characteristics and cost. In choosing one of these materials for an amplifier design, the key is understanding how these different materials characteristics ultimately affect amplifier performance.
The most basic of PCB substrate properties is its permittivity or relative dielectric constant (Dk). For a high frequency amplifier, where signal wavelengths are proportional to the physical lengths of circuit lines and transmission-line structures, the value of the dielectric constant will impact the size of the amplifier’s circuitry, with higher values of Dk resulting in smaller amplifier circuits. For portable designs and products that must be miniaturized, small size can be a positive benefit, although higher-power circuits may need larger circuitry for proper power dissipation. As an example, two members of the RO4000® family of circuit-board laminates from Rogers Corporation, RO4350B™ laminate and RO4360™ laminate, are both hydrocarbon based thermoset-resin dielectric materials but with considerably different Dk values as a result of different filler materials. The RO4350B laminate has a dielectric constant of 3.48 while the RO4360 laminate has a Dk value of 6.15.
For an amplifier, perhaps as important as the Dk value is the consistency of the dielectric constant across the PCB substrate and how the Dk is affected by temperature. In designing an amplifier, one or more power transistors must be impedance matched over a range of desired operating frequencies to the characteristic impedance of the overall circuit or system, usually at 50 Ohms. This impedance transformation requires a circuit-board material that can yield transmission lines with tightly controlled impedances, which requires consistency of the dielectric constant across the length, width, and thickness of the material.
Materials manufacturers provide information on the consistency of their products in terms of a dielectric constant value with an associated tolerance, usually measured in the z-axis (thickness) of the material at some test frequency. For example, the RO4350B laminate has a Dk tolerance of 3.48 ± 0.05 at 10 GHz. The RO4360 laminate has a Dk tolerance of 6.15 ± 0.15 at 2.5 and 10 GHz.
For an amplifier, it is also important that a PCB substrate maintain consistent dielectric constant with temperature. Different substrate materials can be compared in terms of this capability by a parameter known as the thermal coefficient of dielectric constant. It is defined by the amount of change in a material’s dielectric constant in parts per million (ppm) for a given change in temperature in degrees Celsius (°C). Using the RO4350B material again as an example, it has a thermal coefficient of dielectric constant of +50 PPM/°C for ambient temperatures from -50 to +150°C. This means that every 1°C rise in temperature results in an increase of 50 ppm in the value of the material’s dielectric constant.
Another key material parameter to consider when shopping for PCB substrates for an amplifier is dissipation factor or loss tangent. It is a measure of signal energy lost as a result of dissipation through the dielectric material. Ideally, the number for a selected PCB material should be as low as possible to ensure minimal loss in amplifier output power and signal gain. As an example of actual values, the dissipation factor for the RO4350B material is a low 0.0031, with an even lower 0.0030 for the RO4360 material when both are tested at 2.5 GHz.
So far, the factors in considering a PCB material for an amplifier design have focused on the importance of being able to fabricate circuits with consistent impedance. Ensuring that a PCB material’s dielectric constant remains stable across the material’s dimensions and with temperature can help, but what about the effects of high-humidity environments? Many amplifiers designed for cellular networks are mounted in towers or outdoors where they can be exposed to conditions of changing humidity. PCB materials suppliers evaluate their products in terms of a parameter known as moisture absorption. When moisture is absorbed by a substrate, it is essentially adding water, with a dielectric constant of about 73, to a material with a much lower dielectric constant. Depending upon the percentage of moisture absorption, the shift in dielectric constant can be significant. Some PCB laminates, for example, suffer moisture absorption as high as 2 percent which will effectively raise the dielectric constant, resulting in a change in impedance, increase in reflected signals, and reduction in amplifier gain. Substrates engineered to minimize changes in dielectric constant under high-humidity environments are usually characterized by low values of moisture absorption. For example, RO4360 laminate has moisture absorption of only 0.13 percent, even when submerged in warm water for 48 hours.
In terms of controlling the heat generated by a power amplifier, a PCB material’s coefficient of thermal expansion (CTE) and its thermal conductivity can serve as useful parameters when considering different materials for an amplifier design. Materials suppliers measure the CTE in all three of a laminate’s axes (x, y, and z). Ideally, all three values should be as low as possible, denoting minimal physical change in a material with changes in temperature. In the x and y directions, the CTE is usually designed to match that of the copper laminated on the dielectric material (about 17 ppm/°C). For multilayer circuits, in which plated through holes (PTHs) are used to make electrical connections between layers, the CTE in the z-axis (thickness) is critical to ensure good PTH reliability. In general, a material with z-axis CTE of 70 ppm/°C or less will provide good PTH reliability. As an example, Rogers RO4350B laminate has CTEs of 14 and 16 ppm/°C, respectively, in the x and y axes and 35 ppm/°C in the z axis. RO4360 laminate has CTEs of 16.6 and 14.6 ppm/°C, respectively, in the x and y axes and 30 ppm/°C in the z axis.
Thermal conductivity is a critical parameter for truly high-power amplifiers, where a considerable amount of heat must be transferred away from the power transistors to ensure long-term reliability. The thermal conductivity is defined as the watts of power per meter of laminate material per degree Kelvin. Higher values denote better power-handling capabilities for a PCB material. For Rogers RO4350B material, for example, the thermal conductivity is 0.62 W/m/K, while for RO4360 laminate, the thermal conductivity is 0.8 W/m/K.
In addition to the numerous heat- and thermal-related properties which have been discussed and which are important to the choice of PCB materials for power amplifiers, the designer should keep in mind that prolonged exposure in an oxidative environment may cause changes to the dielectric properties of hydrocarbon based PCB materials. The rate of change increases at higher temperature and the net effect of such change is highly dependent on the circuit design. It is recommended that the designer evaluate each material and design combination to determine fitness for use over the entire life of the end product.
For those in Europe, later this month, wishing to learn more about choosing materials for power amplifier designs, don’t forget to visit Rogers Corporation at Booth 106 during the upcoming European Microwave Week 2010 (EuMW 2010, www.eumweek.com), CNIT, Paris, France, September 28-30, 2010. In addition, hear Rogers’ materials specialist John Coonrod present his talk, “High frequency PCBs Using Hybrid and Homogeneous Constructions,” on the use of laminates and prepreg materials for multilayer circuits, at PCB West (www.pcbwest.com), Santa Clara Convention Center, Santa Clara, CA, September 29, 2010.